Non-Patent Document 1, which will be described below, discloses a direct AC power converting apparatus including a clamp circuit. FIG. 9 shows the direct AC power converting apparatus described in Non-Patent Document 1. It is assumed here that an IPM motor is provided on an output side of this direct AC power converting apparatus. When La represents an inductance per phase which corresponds to an average value of effective inductances of the IPM motor, i represents overload current which serves as a reference for interrupting current supply to the IPM motor, Vc represents voltage between both ends of a clamp capacitor, Cc represents electrostatic capacitance of the clamp capacitor, and Vs represents line voltage of a three-phase AC power supply, and when all power stored in an inductor for three phases of the IPM motor is regenerated to the clamp capacitor, the following relational expression is satisfied.
                              [                      Expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            1            2                    ⁢                      La            ⁡                          (                                                i                  2                                +                                                      (                                          i                      2                                        )                                    2                                +                                                      (                                          i                      2                                        )                                    2                                            )                                      =                              1            2                    ⁢                      Cc            ⁡                          (                                                Vc                  2                                -                                                      (                                                                  2                                            ⁢                      Vs                                        )                                    2                                            )                                                          (        1        )            
Therefore, the voltage between both ends of the clamp capacitor is expressed by the following expression.
                              [                      Expression            ⁢                                                  ⁢            2                    ]                ⁢                                                                                      Vc        =                                                            3                2                            ⁢                              La                Cc                            ⁢                              i                2                                      +                          2              ⁢                              Vs                2                                                                        (        2        )            
FIG. 10 shows the relationship between voltage between both ends and electrostatic capacitance of the clamp capacitor, which is based on Expression (2). For example, if the power supply voltage Vs is 400 V, the inductance La is 12 mH, the overload current i is 40 A, and the electrostatic capacitance of the clamp capacitor is 10 μF, the voltage Vc between both ends of the clamp capacitor is approximately 1,800 V. The voltage value exceeds device rating 1,200 V of a transistor and a diode with power supply voltage of 400 V class.
In order to keep the voltage Vc between both ends of the clamp capacitor at approximately 750 V or lower, the electrostatic capacitance of the clamp capacitor needs to be 200 μF or larger from Expression (2) and FIG. 10.
On the other hand, inrush current at power-on increases as the electrostatic capacitance of the clamp capacitor is increased. Here, a series circuit in which a power supply, a reactor, a resistor and a capacitor are connected in series is taken as an example of a series circuit for one phase, where L represents an inductance of the reactor, R represents a resistance value of the resistor, and C represents electrostatic capacitance of the clamp capacitor. Then, a transfer characteristic of output (current) to input (power supply voltage Vs) in the series circuit is expressed by the following expression.
                              [                      Expression            ⁢                                                  ⁢            3                    ]                ⁢                                                                                                G          ⁡                      (            s            )                          =                              ic            Vs                    =                      sC            ⁢                                                  ⁢                                          1                /                LC                                                              s                  2                                +                                  sR                  /                  L                                +                                  1                  /                  LC                                                                                        (        3        )            
The response to step input is expressed by the following expression.
                              [                      Expression            ⁢                                                  ⁢            4                    ]                ⁢                                                                                                G          ⁡                      (            s            )                          =                              sC            ⁢                                                  ⁢                                          1                /                LC                                                              s                  2                                +                                  sR                  /                  L                                +                                  1                  /                  LC                                                      ⁢                          1              s                                =                                    1              /              L                                                      s                2                            +                              sR                /                L                            +                              1                /                LC                                                                        (        4        )            
Here, Expression (4) is subjected to inverse Laplace transform to obtain the response of current assuming that 1/L=D, R/L=E and 1/LC=F, the following expression is derived.
                              [                      Expression            ⁢                                                  ⁢            5                    ]                ⁢                                                                                                i          ⁡                      (            t            )                          =                              D            ω                    ⁢                      ⅇ                          -              α                                ⁢          sin          ⁢                                          ⁢          ω          ⁢                                          ⁢          t                                    (        5        )                                          [                      Expression            ⁢                                                  ⁢            6                    ]                ⁢                                                                                                ω          =                                                                      4                  ⁢                  F                                -                                  E                  2                                                      2                          ,                  σ          =                      E            2                                              (        6        )            
F decreases as the electrostatic capacitance C of the capacitor increases, and D and E remain constant irrespective of the electrostatic capacitance C, and thus ω decreases as the electrostatic capacitance C of the capacitor increases. Accordingly, an amplitude term D/ω excluding attenuation through time increases as the electrostatic capacitance C of the capacitor increases. That is, inrush current increases along with an increase in electrostatic capacitance C of the capacitor.
When the maximum value of current is obtained assuming that a value obtained by differentiating i(t) with respect to time is 0 (i(t)'=0) from Expression (5), the following expression is derived.
                              [                      Expression            ⁢                                                  ⁢            7                    ]                ⁢                                                                                      t        =                              π            -            α                    ω                                    (        7        )            
The maximum value is regarded as inrush current. FIG. 11 shows the relationship between inrush current (i((π−α)/ω)) and the electrostatic capacitance C.
As described above, the voltage between both ends of the clamp capacitor charged with the regenerative current is approximately equal to or lower than 750 V, and accordingly if the electrostatic capacitance of the clamp capacitor is 200 the maximum value (inrush current) of current reaches 150 A from Expressions (6) and (7) and FIG. 11.
In Non-Patent Document 1, for reducing the above-mentioned inrush current and also reducing the voltage between both ends of the clamp capacitor charged with the regenerative current, a discharge circuit is provided in the clamp capacitor. More specifically, the discharge circuit includes a discharge resistor connected in parallel with the clamp capacitor. The inrush current is reduced by reducing the electrostatic capacitance of the clamp capacitor, and charges charged in the clamp capacitor are discharged to the discharge resistor when the voltage between both ends of the clamp capacitor exceeds a predetermined reference voltage due to the regenerative current, whereby the voltage between both ends is suppressed from increasing.
Note that Patent Documents 1 to 4 disclose the technologies related to the present invention.    Non-Patent Document 1: J. Schoenberger, T. Friedli, S. D. Round, J. W. Kolar, “An ultra sparse matrix converter with a novel active clamp circuit”, Proc. of the 4th power conversion conference (PCC '07), pp. 784-791    Patent Document 1: U.S. Pat. No. 6,995,992    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-54947    Patent Document 3: Japanese Patent Application Laid-Open No. 02-65667    Patent Document 4: Japanese Patent Publication No. 62-53918